This invention relates to a beam-index type color image-presentation cathode ray tube.
An index color image-presentation cathode ray tube (hereinafter referred to as an index tube) is unprovided, unlike a shadow-mask tube, with a color switching and selecting electrode, and it is necessary in the index color image-presentation cathode ray tube to excite with electron beams those index-phosphor stripes on a metal-backed tricolor phosphor screen to emit a light, cause an index light signal so obtained to be converted through a transparent light-receiving window on an index tube funnel to an index signal by means of a photoelectric converting element on the outer surface of the funnel, detect the position of the electron beams on the fluorescent screen, and synchronize and modulate a chroma signal to obtain a color reproduction image.
Suppose that the triplet pitch of the color-phosphor stripes is P.sub.T and the pitch of the index-phosphor stripes is P.sub.I. When in this case a color reproduction is effected in an index tube whose P.sub.T /P.sub.I ratio is a non-integral number (normally P.sub.T /P.sub.I =3/2 or 3/4), several starting index strips are arranged on the electron beam scan starting side of the index phosphor stripes to obtain a reference signal (so-called "starting signal) for taking a color synchronization uniformly at each scan starting time; the starting index-phosphor stripes are excited by scanning with electron beams to emit an index signal light; the light is converted through the photoelectric converting element to an electric index signal; and a corresponding pulse is counted so as to obtain a color synchronization. The pitch of the starting index-phosphor stripes is usually three times the pitch P.sub.I of the index stripes i.e. the running index stripes, and the stripe width is substantially P.sub.I /2.
If the triplet pitch P.sub.T becomes small in order to reproduce a fine image, the pitch of the starting index-phosphor stripes naturally becomes small with the result that the stripe width becomes small. For a 20-inch index tube, for example, P.sub.I becomes 0.6 mm at P.sub.T =0.9 and the width of the starting index-phosphor stripes is about 0.9 mm. When a color synchronization is taken using this method, the pulse number of the starting signals is required to be accurately counted. Suppose that an error resulting from, for example, the variation of high voltage and the non-linearity of the deflecting yoke is several % varied momentarily. Then, an erroneous operation will be involved and consequently color synchronization could not be obtained. Thus, a complicated compensation circuit will be required.
If in the index tube any residue etc. of the index phosphor is present in the neighborhood of the starting index phosphor, an unwanted pulse is generated, causing an erroneous operation of the counter circuit and thus failing to effect a normal color reproduction. In order to eliminate undesired pulses a light-impervious film such as graphite and carbon is partially attached by vapor evaporation, or coated, near to the starting index-phosphor.
FIG. 1 is an enlarged cross-sectional view showing a screen structure of a conventional index tube. In this Figure is shown a cross-section when a screen section corresponding to a beginning of the horizontal scanning period is cut in a direction of scanning of electron beams. Reference numerals 1 and 2 show index-phosphor stripes and the index-phosphor stripes 1 in particular show starting index-phosphor stripes. Reference numeral 3 is a metal film, such as aluminium, which serves as a metal back. Reference numeral 4 is a light-impervious material and reference numeral 5 is an arrangement of tri-color (red, green and blue) phosphor stripes. Reference numeral 6 is a face plate. An arrow 7 indicates a direction in which electronic beams come and an arrow 8 indicates a direction in which electron beam scanning is effected. 1A shows the undesired residue of index-phosphor etc. FIG. 3 shows a relation of a color step-out or out of color-synchronization, a cause for defects, resulting from such residue of the undesired index-phosphor. That is, FIG. 3 shows a manner in which, when a color reproduction is effected using a circuit of FIG. 2, the gate ON time after time period T.sub.1 from a horizontal synchronizing pulse of a counter circuit 14 varies relative to the position of each phosphor of the index tube owing to the high voltage variation or the non-linearity of a deflecting yoke. This means that no normal pulse number can be obtained at the counter circuit 14 of FIG. 2. This leads to a color stepout. FIG. 3(A) shows the output waveform of a photoelectric-converting element 9 and FIG. 3(B), the waveforms of a starting index signal which correspond to outputs passing through frequency selection circuits 10 and 12 and waveform shaping circuits 11 and 13. The signal has a predetermined relation to the horizontal synchronizing pulse and it is adapted to be counted by the counter circuit 14 which is operated for a time period T.sub.2 after delay time period T.sub.1 from the horizontal synchronizing pulse. When the horizontal synchronizing pulse leads or lags due to the above-mentioned voltage variation and the non-linearity of the deflecting yoke, count errors occur as shown in FIGS. 3(C) and 3(D), causing a color step-out.
In FIG. 2, numeral 15 shows a horizontal synchronizing circuit; numeral 16, a pulse gate circuit; numeral 17, a frequency converting circuit; numeral 18, a phase correction circuit; numeral 19, a color gate circuit; numeral 20, a color mixing circuit, and numeral 21, an index tube.